The present invention generally relates to gas turbine engine systems and, more particularly, to high pressure turbine (HPT) blade outer air seal (BOAS) assemblies, also known as turbine shroud assemblies.
Turbine shroud assemblies have been used extensively in gas turbine engines. The turbine shroud assembly may be positioned immediately downstream of an HPT nozzle. The turbine shroud assembly may surround a HPT rotor and may define an outer boundary of a high temperature gas flow path through the HPT. During engine operation, exposure to the high temperature gas flow may result in failure of the turbine shroud components. Due to the differing expansion of rotor and turbine shroud assembly components, it may also result in contact between the turbine shroud assembly and the blade tips of the rotor. A small amount of cooling air from a compressor may be used to decrease some of the adverse effects of the high temperature gas flow.
Minimizing the amount of air necessary to cool the turbine shroud assembly is desirable because engine efficiency decreases as the amount of cooling air increases. Methods for minimizing the cooling air necessary may include decreasing cooling air leakage from the assembly or reducing the cooling needs of the system by increasing the effectiveness of the cooling scheme.
Turbine shroud assemblies have experienced significant distress due to a lack of robust sealing of the assembly. This leakage may result in a significant reduction in the cooling cavity pressure (and back flow margin), which can result in hot gas ingestion and distress in the hardware. Back flow margin is the ratio of the difference between the shroud cooling cavity pressure and the flow path pressure to the flow path pressure. If the back flow margin of the assembly becomes negative (or for some designs even a low positive number), hot flow path gas may ingest into portions of the shroud and may cause significant distress. The challenge in maintaining good back flow margin is due to the difficulty in sealing the various leak paths that allow the cooling air to escape from the shroud cooling cavity.
Several methods of reducing cooling air leakage have been disclosed. These methods include the use of labyrinth type seals and metallic platform seals. Unfortunately, labyrinth seals are not suitable for some applications, and the metallic platform seals, which are secured in machined grooves in the sides of the segments, may fail in the operating environment of some engines. In addition, assembly technicians may cut themselves on the small, sharp metallic platform seals.
Methods of reducing system cooling needs have also been disclosed. Manufacturing the assembly components from more robust materials and utilizing thermal barrier coatings (TBC) have been described. Designs that utilize TBC to keep the shrouds insulated from the hot flow path gas can experience delamination of the TBC, which may result in shroud distress, which may result in large turbine blade tip clearances. The subsequent increase in turbine blade tip clearance significantly hurts fuel consumption and also results in an increase in turbine inlet temperature, which further distresses the hardware.
Methods of increasing the effectiveness of cooling configurations have been disclosed. In one method complex arrays of film cooling holes have been drilled into shroud segments. Although this results in increased cooling of the turbine shroud assembly, all edges of the shroud segments may not be sufficiently cooled and system integrity may suffer.
Turbine shroud assemblies having increased cooling of the shroud segment edges have been disclosed in U.S. Pat. No. 6,270,311. This invention utilizes an interlocking hook/shelf on the ends of the segments in conjunction with conventional feather seals and slots to produce an end gap seal between the adjacent circumferential segments. In addition, this invention uses film cooling holes to reinforce cooling at the sides of the segment. Although cooling of the shroud segment edges is increased, the metallic feather seals may suffer distress at higher operating temperatures, which may result in a loss of back flow margin to the assembly.
Another turbine shroud assembly has been disclosed in U.S. patent application No. 2003/0133790. This invention requires that the turbine shroud segment and the shroud segment hanger both are segmented arcs. This invention relies on tight tolerances to minimize leakage of the assembly. Sliding the turbine shroud segment into the shroud segment hanger requires tight tolerances to keep the air seal along the forward and aft hooks. Unfortunately, the tolerances needed may result in increased production costs of the turbine shroud assembly. In addition, fine-tuning of the thermal expansion of the forward and aft hangers may not be possible. Further, to change cooling flows, this invention requires that every shroud segment hanger be reworked.
Turbine shroud assemblies having increased cooling efficiency have been disclosed in U.S. Pat. No. 5,188,506. This design incorporates a rope seal radially outside of the segment forward hook to reduce the leakage of cooling air through the forward hook region of the shroud support. Unfortunately, the forward hook of this assembly may be exposed to hot ingested flow path air. Also, fine tuning of the thermal growth of the forward hanger may not be possible. Further, the disclosed assembly may not allow for sufficient axial motion of the shroud segment as the pressure loads move the segment aft. Because turbine inlet temperatures will continue to rise to achieve greater thrust to weight capability and improved fuel consumption, still further improvements are needed.
As can be seen, there is a need for improved turbine shroud assemblies. Additionally, assemblies are needed wherein cooling air flow is minimized while allowing for increased gas flow temperatures. Further, turbine shroud assemblies are needed wherein blade tip clearance is decreased. Moreover, turbine shroud assemblies having improved cooling schemes to the forward and aft hangers are needed. Also, assemblies are needed that have reduced cooling air leakage and improved shroud segment sealing.